Friction with balls rolling down a funnel
I  don't know ... in theory right curvature (what is right?), enough weight and initial speed should work, but didn't for me either. It is loosing speed too quickly. Some low Friction helped a bit, so its not such slippery. 


add Vortex force field, Strenght 20, Inflow -2 and 
lower Gravity to -4 


to make it work. Even this is not going in spiral, but in oval. I'm not happy about that, but probably better something than nothing.




Also there seems to be some collision margin, but I checked sphere and collider, there is almost notnihg.





Notes: Initial ball velocity is given by animation (first 10 frames). To make it work for riggid body you have to ad two keyframes to two animate checkbox frame zero on/ frame 10 off (Physics Properties > Riggid Body > Settings > Animated)